Andalusite is a polymorph of kyanite and sillimanite. These three minerals share the same formula, but are stable at different P/T conditions.
Physical Properties |
|
Chemical formula | Al2SiO5 |
Class | Nesosilicate Isolated tetrahedra |
Crystal system | Orthorhombic |
Habit | Stubby to elongate square prisms Individual crystals may be rounded Massive Granular |
Color | Brown to red |
Hardness | 7.5 |
Specific gravity | 3.18 |
Cleavage | Rarely seen, good {110}, poor (100) |
Fracture | Subconchoidal |
Luster | Vitreous |
Transparency | Transparent to translucent |
Streak | White |
Optical Properties |
|
PPL | Usually colorless May be weakly colored and pleochroic |
XPL | 1st order yellows (pale tan) May be euhedral with a square outline and penetration twins or cross pattern |
δ | 0.010 |
Twinning | Penetration twins or Maltese cross pattern |
after Perkins, 345 |
Andalusite in Hand Sample
Four small subhedral to euhedral andalusite crystals
Andalusite in a rock
Andalusite in Thin Section
Thin Section GigaPans
Andalusite in slate, plane polars
Andalusite in slate, crossed polars
![](http://blogs.nvcc.edu/mineralogy/files/2017/11/50HD4277-1024x768.jpg)
![](http://blogs.nvcc.edu/mineralogy/files/2017/11/50HD4279-1024x768.jpg)
Further Reading
Andalusite at webmineral.com
Andalusite at mindat.org
Bonus Content
The song that gets stuck in your friendly author’s head every time she encounters andalusite.